Proving the Business Case for the Internet of Things

Analog Devices chip aids vital signs monitoring

Steve Rogerson
June 6, 2019



Massachusetts-based electronics firm Analog Devices has announced an electrochemical and impedance measurement front end that can be used in vital sign monitoring devices and intelligent electrochemical sensors.
 
The AD5940 analogue front end incorporates potentiostat and electrochemical impedance spectroscopy (EIS) functionality on a single chip, allowing for sensor measurement in time and frequency domains. The device has integrated hardware accelerators for sensor diagnostics and low noise for accurate sensor measurements, and is designed for wearable always-on applications.
 
Compared with traditional discrete products that pose limitations and require multiple ICs to achieve similar performance, this single chip is said to have advantages in terms of system accuracy and size flexibility to measure two-, three- and four-lead electrochemical sensors.
 
It is suitable for applications where high precision biological and chemical sensing is mission-critical, such as industrial gas sensing, liquid analysis, material sensing, vital signs monitoring, impedance spectroscopy and disease management.
 
The analogue front end combines levels of integration and performance for potentiostat and impedance-based electrochemical sensor managements. The on-chip potentiostat allows for a host of standard electrochemical-based measurement techniques, such as amperometric, voltametric or impedance measurements.
 
The device is designed to be used in healthcare-related bio-impedance systems for skin impedance and body impedance measurements, and to work with the AD8233 AFE in a complete bioelectric and biopotential measurement system.
 
The chip can measure voltage, current and impedance. The device consists of two potentiostat loops, a low bandwidth loop with the ability to generate AC signals up to 200Hz and a high bandwidth loop with ability to generate AC signals up to 200kHz. The potentiostat consumes 6.5µA in biased mode.
 
The AD5940 measurement channel has a 16bit, 800kS/s, multichannel successive approximation register, analogue-to-digital converter (ADC) with input buffers, a built in anti-alias filter and a programmable gain amplifier.
 
The ADC has an input voltage range of ±1.35V. An input mux before the ADC allows the user to select an input channel for measurement. These input channels include multiple external current and voltage inputs, and internal voltage channels. The internal channels enable on chip diagnostic measurements of the internal supply voltages, die temperature and reference voltages.
 
The measurement blocks can be controlled via direct register writes through the SPI serial peripheral interface or by using a pre-programmable sequencer, which provides autonomous control of the AFE chip. And 6kbyte of static random access memory is partitioned for a deep data first in, first out (FIFO) and command memory. Measurement commands are stored in the command memory and measurement results are stored in the data FIFO. A number of FIFO related interrupts are available to indicate the state of the FIFO.